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Bacterial succession in bioheap leaching

C.L. Brierley

Brierley Consultancy LLC, P.O. Box 260012, Highlands Ranch, Colorado 80163-0012, USA

Bioheap leaching of secondary copper ores is applied commercially at operations in Chile and Australia. Bioheap leaching of sulfidic refractory gold ores has been demonstrated at large scale. There is limited comprehension of what actually occurs microbiologically in full-scale bioheap operations, despite the commercial achievement of copper ore bioheap leaching and the anticipated technical and commercial success of gold ore bioheap leaching. Copper bioheaps are typically inoculated with the bacteria contained in the raffinate, whereas, sulfidic refractory gold ore bioheaps can be inoculated with bacteria developed in a separate reactor. Chemical and physical conditions within bioheaps change radically from the time the bioheap is stacked and inoculated until bioleaching is completed. Redox, acidity, temperature, oxygen and solution chemistry conditions can vary widely during the oxidation period. Such conditions likely select for microorganisms or may, in fact, effect a succession of organisms in portions of the bioheap. Bioheap solutions are recycled and constituent build-up over time also affects the microbiology. Heterotrophic microorganisms may also play some role in bioheap leaching. Understanding the microbiology of bioheaps is key to advancing commercial bioheap applications. Such knowledge will increase the ore types as well as the diversity of mineral deposits that can be processed by bioheap technology. It will also enable better control of conditions to improve leach rates, metal recoveries and costs. This paper briefly explains commercial practices, describes chemical, physical and microbiological monitoring of bioheap, considers conditions that control microbial populations in bioheaps, and examines the types of ore deposits that could be bioleached, if the microbiology was elucidated.

I. INTRODUCTION

Commercial bioleaching began in the 1950s with dump leaching, a process that releases copper from vast quantities of submarginal-grade primary and secondary copper sulfide materials. Today dump leaching remains a vital process for the copper industry. In the last decade a coupling of dump leaching, copper oxide heap leaching, and industrial microbiology has yielded a successful commercial process for bioheap leaching secondary copper (primarily chalcocite and covellite) ores and technical demonstration of sulfidic refractory precious metal ore bioheap leaching. Bioheap leaching is a simple yet robust process offering capital and operating cost advantages and environmental benefits. The proliferation of bioheap leach operations (Table 1) during the last five years attests to the good performance and profitability of the technology. In addition to the commercial plants, bioheap leaching of refractory sulfidic gold ores was evaluated by Newmont Mining Corporation during a large-scale (800,000 t) demonstration project at the Company's Carlin, Nevada operations.

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Table 1 Commercial bioheap leach operations

Operation Location Ore processed/ product

Lo Aguirre Chile Chalcocite/Cu Quebrada Blanca Chile Chalcocite/Cu Cerro Colorado Chile Chalcocite/Cu

Size Status (t of ore/day) 16,000 In closure 17,300 In operation 16,000 In operation

Ivan-Zar Chile Chalcocite/Cu 2,000 In operation Andacollo Chile Chalcocite/Cu 10,000 In operation Dos Amigos Chile Chalcocite/Cu 3,000 In operation Zaldivar Chile Chalcocite/Cu -~20,000 In operation Mt. Leyshon Australia Transitional supergene/ 1,370 Ceased operation

Cu, Au Girilambone Australia Chalcocite/Cu 16,000 In operation

Notwithstanding the commercial success of secondary copper ore bioheap leaching and favorable technical demonstration of sulfidic refractory gold bioheap leaching, little is known about the microbiology of bioheap leaching. This paper describes the practice of commercial- scale bioheap leaching, reviews factors affecting bioheap microbiology, and relates how the microbiological aspects are managed in commercial operations. The principal aim of this presentation is to survey what is and what is not known about the microbiology and clarify why understanding the microbiology is integral to advancing this technology.

2. FULL-SCALE BIOHEAP LEACHING PRACTICES

Commercial practices for secondary copper ore bioheap leaching are briefly described. The proposed method for full-scale bioheap leaching of sulfidic refractory precious metal ores is explained.

2.1 Secondary copper ore bioheap leaching Mined copper ores are crushed to an optimum particle size. The crushed ore is mixed with

sulfuric acid in an agglomerating device to consolidate the fines with the coarser ore particles and precondition the ore for bacterial development. Water or raffinate (effluent from the solvent extraction-electrowinning circuit) is added to optimize the moisture content for good agglomerate formation. If the ore is not too acid consuming, the acid requirement to precondition the ore can be met by agglomerating with raffinate. Raffinate ot~en contains a small population of bacteria, which inoculate the ore. The agglomerated and preconditioned copper ore is conveyed to the leach area where it is stacked 6 to 10 m high on a lined pad or on top of previously leached ore. Plastic piping with ventilation holes is placed on the pad or liit to supply air to the bacteria during leaching. Aeration of the bioheap is initiated soon atter stacking the agglomerated ore. Low-pressure fans supply air to the ventilation system under the ore. The bioheap is irrigated with raffinate at an application rate that does not cause saturation. Pregnant leach solution (effluent containing copper) is collected at the base of the bioheaps and directed to a SX/EW (solvent extraction/electrowinning) circuit for copper recovery. The raffinate (barren solution) is returned to the bioheap for irrigation. Leach times

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vary among the operations, but is typically in the 200-day range for secondary copper ores. Copper recoveries also vary with 75 to 85% recoveries achieved at most operations. A recent book chapter details the bioheap leaching of secondary copper ores (1).

2.2 Sulfidic refractory precious metal ore bioheap leaching Since sulfidic refractory precious metal ore bioleaching has not been applied commercially,

the actual full-scale practice has not been completely documented. However, demonstration and pilot plant trials, which have been conducted, project the design and operation of full- scale plants. There are many similarities between bioheap leaching of copper ores and sulfidic refractory gold ores; however, there are also some consequential differences. Like copper ores, the sulfidic precious metal ores are crushed to an optimum particle size that is determined in laboratory tests. If the ore is highly acid consuming, concentrated sulfuric acid is used during agglomeration to precondition the ore for bacterial development. If the ore is not highly acid consuming, a bacterial culture may be used in the agglomeration step to meet the acid demand of the ore, add moisture and inoculate the bioheap. After agglomeration, the ore is stacked on a dynamic (ON/OFF) pad with a coarse gravel bed. Aeration lines are buried in the coarse gravel bed. Aeration is initiated when the pad is fully stacked. The stacked precious metal ore bioheap is irrigated with a mixed culture of iron-oxidizing bacteria, which are usually grown on ferrous sulfate. Effluent from the bioheap is also recycled for irrigation. The biooxidation time is dependent on the mineralogy, the amount of sulfide-sulfur requiring oxidation and other factors, such as bioheap temperature. The biooxidized gold ore is usually rinsed with flesh water to remove constituents that consume lime and cyanide. After rinsing, the biooxidized ore is removed from the pad, mixed with lime and re-stacked on a permanent pad for leaching with cyanide or other lixiviant. The inoculation and bioheap leaching of sulfidic refractory gold ores are detailed by J.A. Brierley (2).

3. MONITORING BIOHEAPS

The extent of ore bioheap monitoring varies widely from operation to operation and also changes with the maturity and performance of the bioheap operation. A greater degree of monitoring takes place when the bioheap operation is in start-up or if problems arise during operation. The following are monitoring techniques employed throughout the industry, although rarely are all of these approaches applied at any singular operation.

The PLS (pregnant leach solution) from secondary copper bioheaps and effluent from sulfidic refractory gold ore bioheaps are analyzed for pH, redox potential, acidity, total iron concentration, ferrous iron levels, arsenic (for arsenic-bearing ores) and copper (for copper bioheaps), pH and acidity measurements indicate the extent of acid conditioning of the bioheap and provide insight into the oxidation of pyrite. Redox potential, iron concentrations, ferrous:ferric ratios, and arsenic concentrations provide information on pyrite and other iron- bearing mineral dissolution and the performance of the iron-oxidizing bacteria.

Periodically samples of ore are collected at different depths and locations throughout the copper bioheap leach period and are assayed for residual copper. This provides inventory measurements as well as performance information. Other constituents, such as sulfide-sulfur, arsenic and iron, may be assayed to determine the extent of sulfide oxidation, the minerals that are oxidizing and the respective rates of oxidation. In refractory sulfidic gold bioheaps, the

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solids are subject to bottle roll cyanidation to confirm rate and extent of precious metal recoveries as related to sulfide-sulfur oxidation.

Temperature measurements at various depths and locations throughout the bioheap provide information on pyrite oxidation. High temperatures attest to active pyrite oxidation.

Oxygen measurements at various depths indicate whether sufficient aeration is being applied, and, when oxygen measurements are coupled with solids analyses, these data may point to active sulfide mineral oxidation.

Oxygen uptake measurements of solid samples, PLS and raffinate using a respirometer provide information on the activity of the bacteria. The principal disadvantage of this technique is the cost of the equipment.

Bacterial counts can be made with a fair degree of accuracy in liquids, but the data are unreliable for solids. Therefore, indirect measurements, such as oxygen uptake, provide more accurate information. The mining industry is just becoming aware of the availability and potential value of molecular methods in determining the microbial populations in bioheaps (3). Little molecular biology data are available from commercial bioheap operations.

The validity of these monitoring techniques is predicated on obtaining a sufficient number of representative samples and having accurate baseline (i.e. pre-bioleach) parameters. Moreover, the most valuable information is gained by correlating the chemical and physical conditions of the bioheap and the specific samples with microbial assays and molecular biology data. Notwithstanding the care taken in sampling and analysis of bioheap solids, conclusions drawn from analyses of these samples may not necessarily reflect what is actually happening in the bulk of the bioheap. The error is related to the inherent lack of uniformity of the ore in bioheaps.

4. ORE BIOHEAP MICROBIOLOGY AND INFLUENCING FACTORS

The microbiology of ore bioheaps from start-up through decommissioning is neither well studied nor fully understood. Nonetheless, what is recognized are some of the determinants that influence the numbers and types of microorganisms that colonize and function during the existence of a bioheap.

4.1 Acid consumption of the ore Ores may contain carbonate minerals, clays or both that consume acid. Laboratory testing

confirms the amount of sulfuric acid that must be added to condition the ore for optimum colonization by the mesophilic, acidophilic chemolithotrophic bacteria. However, in the case of chalcocite ores not all of the acid to achieve the optimum pH is added during agglomeration, because the remaining acid demand is met by the raffinate. Some sulfidic refractory gold ores may also be stacked at a higher pH, because of the nature of the ore. In these situations the pH conditions are not optimum for mesophilic, iron-oxidizing bacteria, yet, sulfide oxidation eventually initiates. Are there other bacteria, functioning at a higher pH, that initiate the oxidation? What is the bacterial succession? How long is required before the ore is conditioned for Thiobacillusferrooxidans and related bacteria to develop and perform at maximum capacity?

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4.2 Sulfide content and rate of oxidation Some ores have a high sulfide-sulfur content (for example 5-10%) that generates

considerable heat during oxidation. Portions of bioheaps can heat to 75~ (4, 5) and perhaps higher depending on the amount of sulfide-sulfur, the rate of oxidation and heat losses through evaporation and other mechanisms. Although the bioheap may have been inoculated with a mixed culture of Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and other related bacteria, obviously those portions of the bioheap that exceed the maximum temperature range for these bacteria have a different microbial flora. What organisms colonize the higher temperature portions of the bioheap? How long does it take for these organisms to establish themselves? In what numbers are they present? Is there a succession of thermophilic bacteria? Do extreme thermophiles, such as Sulfolobus and Acidianus species, colonize bioheaps? Are the thermophiles beneficial in bioheaps? Are higher temperatures beneficial or detrimental to sulfidic refractory gold bioheap leaching? A sulfidic refractory gold ore column study by Olson et al. (6) showed that sulfide oxidation rates of thermophiles were similar to the mesophilic bacteria, but precipitate build-up at the interface between ambient and high temperature zones could reduce permeability. Clearly, controlled studies are needed to provide answers to these and other questions.

4.3 Solution chemistry The chemistry of the leach solution in bioheaps is the result of the quality of the water used

to make up the leach solution, the mineralogy of the ore and equilibrium conditions of the solution. In some geographical locations the local water quality may be poor with relatively high concentrations of CI, NO3-, TDS or all three. Simply getting bacteria to grow under such conditions can be challenging. When leaching starts, iron concentrations rapidly build in solution. The ultimate iron concentration in the solution is dependent on the ore, temperatures in the bioheap, and aeration (1). Soluble iron concentrations in excess of 20 - 30 g/L are possible. If the ore contains arsenic minerals, solution arsenic concentrations increase rapidly. Many bioheaps are operated in "closed cycle"; no bleed stream, other than solution retained in bioheaps that are taken off-line, is employed to remove heavy metals, high sulfate concentrations, high TDS, and potentially toxic constituents. Therefore, when equilibrium is exceeded, massive precipitation of iron and arsenic takes place in the bioheap. The nature of these precipitates that form under a variety of temperature conditions in the bioheap is not well studied, nor is the affect of these precipitates on mineral's oxidation, lime and cyanide consumption, and precious metal recoveries elucidated.

Virtually nothing is known about the microbiological changes that must occur during the build-up and precipitation of various constituents? Are there changes in the bacterial flora over time or do the existing bacteria adapt? Do the bacteria play a role in the precipitation of iron salts or influence the characteristics of the salts?

4.4 Aeration and oxygen concentrations Bioheaps are aerated, but oxygen levels throughout the heap vary widely based on

permeability, oxidation rates, sulfide-sulfur content, bacterial populations and other factors that not well understood. Do the types of bacteria vary among areas of differing oxygen levels? Are there fewer bacteria in oxygen depleted areas or does the same population exist, but just oxidize at a slower rate? Is it possible that bacteria capable of using alternative electron acceptors colonize portions of the bioheap that have limited oxygen? If so, are they important in bioheap leaching?

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4.5 Nutrients The addition of NH4 + and PO4 3" is limited because of cost of these reagents and the risk of

precipitating these nutrients as ammonium jarosite and potassium phosphate. In many bioheaps there are sufficient nutrients to sustain adequate populations of the mesophilic, sulfur- and iron-oxidizing bacteria. Does limiting the nutrients select for any particular population in the bioheap and, if so, is this the fight population? Does this action in any way control the functioning of the bacteria? How does nutrient limitation affect colonization by the moderate and possibly the extreme thermophiles?

Certainly unanswered are questions related to what other types of bacteria develop in bioheaps. Where tertiary wastewater, containing dissolved organics, has been used as make-up water in the bioheap circuit, fungi have grown. This caused pipes to plug and raised concerns about fouling of the SX circuit. This would suggest that a complex microflora could or does develop in bioheaps. What are these other microorganisms and what do they do in bioheaps? Are they beneficial or detrimental to the commercial process?

5. WHY IS MICROBIOLOGY IMPORTANT TO ORE BIOHEAP LEACHING?

Apart from the obvious answer that bacteria are responsible for the oxidation of the sulfide minerals, a better understanding of the microbiology is paramount to future development of bioheap technology. We must comprehend the microbiology to advance the technology, expand the use of the process to a broader spectrum of base and precious metals ores, lower capital and operating costs and improve metal recoveries. Table 2 offers examples where better understanding of bioheap microbiology would advance the technology and its application.

Table 2 Advancements in ore bioheap applications based on microbiology

Microbiological Advancement Potential Benefit to Ore Bioheap Leaching Employ bacteria that initiate oxidation at Allow effective bioheap leaching of high- higher pH values and condition ore for carbonate ores conventional leaching bacteria

Comprehend the adaptation or succession process of the bacteria to the changing character of the leach solutions

Allow use of poorer quality water for make- up of the leach solutions and closed-circuit operation of the bioheaps, reducing capital and operating costs

Understand succession ofthermophiles in bioheaps, interpret their function and define their effects on minerals and precipitates

Extend bioheap leaching to more refractory ores, enhance metal recoveries and decrease leach time, which reduces costs

Describe heterotrophic and chemolithotrophic microflora in bioheaps as related to nutrients and function

Define conditions that could optimize bioheap performance, reducing cost

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6. SUMMARY

Bioheap leaching of secondary copper sulfide and sulfidic refractory gold ores has advanced despite a dearth of knowledge about the bacteria that initially colonize the bioheap and evolve over time and under harsh conditions. The definitive action of the microorganisms in precipitation reactions and solution chemistry is unclear. A more accurate comprehension of the microbiology would almost certainly lead to bioheap leaching of a broader spectrum of ore deposits and to more refractory base metal ores. Explicit microbiological understanding would assuredly reduce both capital and operating costs by allowing use of microorganisms to generate optimum operating conditions rather than employing chemicals to control solution chemistry and precipitation reactions.

REFERENCES

1. H. Schnell, D.E. Rawlings (ed.), Biomining: Theory, Microbes and Industrial Processes, Springer-Verlag and Landes Bioscience, Berlin, 1997.

2. J.A. Brierley, D.E. Rawlings (ed.), Biomining: Theory, Microbes and Industrial Processes, Springer-Verlag and Landes Bioscience, Berlin, 1997.

3. C.A. Jerez, D.E. Rawlings (ed.), Biomining: Theory, Microbes and Industrial Processes, Springer-Verlag and Landes Bioscience, Berlin, 1997.

4. J.A. Brierley, IBS/BIOMINE 97, Australian Mineral Foundation, Glenside, Australia, 1997.

5. M.L. Shutey-McCann, F.P. Sawyer, T. Logan, A.J. Schindler, and R.M. Perry, D.M. Hausen (ed.), Global Exploration of Heap Leachable Gold Deposits, The Minerals, Metals and Materials Society, Warrendale, Pennsylvania, 1997.

6. G.J. Olson, T.R. Clark, and J. Kelso, Randol Gold and Silver Forum '98, Randol International, Golden, Colorado, 1998.